WORCESTER — For the half-dozen times a month Bob Hebb heads into Worcester from his hometown of Ayer, he makes sure to head over to Shrewsbury Street.

His destination is Mac’s Diner, where a large kettle of soup usually beckons and the stools are filled with patrons he knows by name.

But yesterday, Mr. Hebb arrived at his favorite restaurant only to learn that an overnight fire had damaged and closed the business.

The owners of the restaurant — which dates to 1931 — are unsure when they’ll reopen.

“You have never eaten in here? They have a kettle of soup that is about this high,” Mr. Hebb said, holding his hands a couple of feet apart. “I don’t know where I’m going to eat.”

As Mr. Hebb was left wondering where he’d get his usual soup and a sausage sandwich — made on the diner’s homemade bread — a crew of city Department of Public Works and Parks workers headed to the entrance at 185 Shrewsbury St.

Mr. Hebb soon informed them of the situation: “It’s closed. There was a fire.”

About noon yesterday, owner Chris McMahon of Holden walked around inside the diner and assessed the damage. Mac’s is said to be the oldest diner in the city.

He doesn’t know when it will reopen.

“I couldn’t even guess. I’m at the mercy of the Fire and Building departments,” he said. “Hopefully not that long. I have to make a living.”

The fire started about 12:30 a.m. in a storage area in the rear of the building, where there are freezers and refrigerators.

Mr. McMahon said the cause of the fire appears to be electrical but fire officials have not yet determined the cause.

Firefighters broke through a front window and doused the flames with water.

The upper diner area was damaged as well as a side area. The acrid smell of burnt wood replaced the normal smell of home-cooked meals yesterday afternoon.

The damage could have been worse, but the diner’s concrete and brick walls didn’t give the fire much to feed on. Holden police knocked on Mr. McMahon’s door early yesterday morning to notify him about the fire.

He raced down to Shrewsbury Street.

“When I first showed up here, I was in total shock,” he said. “I have come to assess it. It’s manageable, but it is definitely going to set us back.”

“In 78 years, we’ve never had a fire here,” Mr. McMahon, 31, said.

“We plan to fix it as soon as we can because we are all going to be out of money.”

Customers continually called the diner yesterday asking if the owners needed help and to say they were sorry. The diner is normally open Monday through Friday for lunch, with dinner also served Thursday through Saturday.

Many customers favor Mac’s because of its BYOB standing.

“April, May and June are our busy season,” Mr. McMahon said.